Blood cancer is one of the leading causes of death in the US. The NCI estimates 123,180 new cases and 44,080 deaths from leukemia and lymphomas in 2014. Hematopoietic stem cell (HSC) transplantation remains the only cure for these hematological malignancies. But even after transplantation with a curative intent, up to 50% of patients die within the first 3 years. Success of cell therapy lies upon many factors among which, choosing the right cell source, is paramount. However, studying HSC heterogeneity is a difficult challenge because conventional isolation of stem cells using antibody recognition of cell surface markers can only yield an impure population. Attempts have been made to investigate the clonal behavior of HSCs using single cell transplantation or tracking individual HSCs using viral integration of barcoded DNAs. However, single cell transplantation cannot study clonal competition, and viral labeling of DNA requires ex vivo manipulation of cells, which may alter cell properties due to the artificial cell culture conditions. We developed a new multi-fluorescent transgenic mouse model, named `HUe' mouse, to study the clonal dynamics of hematopoietic cells in vivo. This mouse permits cell labeling by a range of fluorescent tags after stochastic rearrangement of conditionally expressed fluorophores. The advantage is that flow cytometric quantification could be readily performed and cells could be isolated for molecular profiling as well as functional assessment. This system allows tracking of a large number of clones at the single cell level in vivo, avoiding ex vivo manipulation of cells and enables us to study HSC clonal behavior in an unprecedented manner. We propose to use this new tool to address the following aims with the long-term goal of understanding fundamental HSC behavior and improving therapeutic stem cell selection/manipulation: aim (1) defining the epigenetic landscape that confer stem cell resistance to stress/injury, and aim (2) determining the epigenetic signatures that control specific lineage fate of stem cell. Completion of these studies will greatly advance our understanding of stem cell subtypes, their functional differences, and unique epigenetic regulation of cell behavior. These studies are the first to define the molecular features that defines a specific stem cell function and for prospective isolation of stem cell subtypes. These studies will provide insights to enhance the success of HSC transplant therapies using patient-tailored stem cell subtypes. Therefore, this proposal is in alliance with the mission of NHLBI in advancing stem cell correction and improving the survival of transplant patients inflicted with hematological diseases.